1. Field of the Disclosure
Embodiments disclosed herein relate generally to a cathodic protection apparatus and method. In particular, embodiments disclosed herein relate to passive cathodic systems.
2. Background Art
Cathodic protection is an electrical method for mitigating corrosion of metallic structures, particularly metallic structures immersed in electrolytes, such as seawater. Marine equipment and structures, particularly those used for offshore oil and gas exploration and production, have long been protected by cathodic protection (“CP”) systems, including both active (“impressed current”) systems using an electrical current source, and so-called passive systems, which typically employ sacrificial anodes of metals which are less noble than the protected equipment and structures. So-called “hybrid” cathodic protection systems may employ elements of both active and passive systems.
Marine equipment and structures that are deployed at or near the seabed, such as subsea blowout preventers (“BOPs”), drilling and production riser pipes, production trees, valves, manifolds, templates and associated piping and pipelines, may typically be protected by passive cathodic protection systems, mainly because of the difficulty and expense of maintaining an impressed current on equipment that may be a mile or more below the ocean surface.
For passive cathodic protection systems, the offshore oil and gas industry has, over time, effectively standardized on sacrificial anodes made from aluminum-zinc-indium alloys, which in seawater may produce a cathodic potential on the order of −1.0 volts (commonly expressed as −1000 millivolts, or mV) referenced to a standard silver/silver chloride (Ag/AgCl) electrode. Anodes of aluminum-zinc-indium alloys typically provide a good balance of cathodic potential, current, economy, and long life in seawater. In addition, anodes of aluminum-zinc-indium alloys have good structural strength in use, relatively uniform consumption across the surface of the anode, and good shelf life in air.
However, as protective coatings have steadily improved, as oil and gas exploration and production has gone into deeper water depths, and as equipment has been constructed of higher-strength steels to meet higher pressure requirements, it has been discovered that standard offshore oilfield passive cathodic protection systems, including those using aluminum-zinc-indium sacrificial anodes, may have several issues. For example, it may be difficult to accurately predict the exact net potential of a passive CP system in marine service, especially proximate the seabed; it requires, for example, a complete understanding of the properties of the electrolyte (seawater) in the environs of the system, the total cathodic area of the protected equipment or structure, and the properties of any coatings on the protected structure.
In addition, in a related issue, it may be difficult to predict the current density that will be achieved by a marine CP system, especially over time as, for example, applied protective coatings or a calcareous layer wear away, or, in the case of mobile offshore drilling units (or MODUs, such as jack-ups, drillships and semi-submersibles) as the marine conditions (such as water depth, water temperature, current velocity, etc.) change significantly from one drilling location to another half a world away. For example, if significant flaws develop in a coating on cathodically protected equipment, a CP system may operate at a lower current density than anticipated. Alternatively, if the paint on a protected structure is thicker or of higher electrical resistance than expected, a CP system may exhibit a higher current density than contemplated, and consequently a higher cathodic potential than desired, which may increase the possibility of deleterious hydrogen embrittlement of the protected structure, particularly for equipment made from high strengths steels (for example, with yield strengths above 700 MPa or about 100,000 psi).
Guidelines for the required current density induced by a passive cathodic protection system typically include a “safety factor” of at least 25 percent. Such a “safety factor” may add considerable weight and expense to the protected structure; or the excess “safety factor” anodes will be sacrificed along with all the other anodes, and will not be available later to, for example, extend the life of the cathodic protection system. In addition, an optimum marine cathodic protection system may require a “potential profile” over time; for example, an initial large negative potential, on the order of −900 mV, to quickly build-up a dense layer of calcareous deposits on the protected structure, and then a much smaller negative potential for “maintenance” of the cathodic protection. While such a “potential profile” may be easily and accurately achieved with an impressed current system simply by adjusting the voltage of the active current source over time, it is extremely difficult, using prior art devices or methods, to accurately adjust the potential of a passive CP system, particularly at or near the seabed.
One prior art approach to controlling cathodic potential of a passive CP system, especially to prevent hydrogen embrittlement, has been to change the composition of the sacrificial anodes to reduce their open-circuit potentials. For example, while commonly used aluminum-zinc-indium anodes may typically have an open-circuit potential of about −1000 to −1050 millivolts, so-called “low voltage” anodes (such as, for example, aluminum-gallium anodes commercially available from, for example, Norton Corrosion Limited of Woodinville, Wash.) may have an open-circuit potential of about −800 millivolts; such low-voltage anodes are generally not capable of polarizing a structure to potentials at which hydrogen embrittlement is a significant risk. This “low voltage” approach has the disadvantages that the cathodic potentials and current are not adjustable in situ, that it requires specialized anodes specifically for areas of protected structures which may be at high risk of hydrogen embrittlement (as opposed to, for example, adjusting the potential of a standard anode in the same service), and that the low voltage anodes required may not be sufficiently mechanically strong in service or have adequate shelf life in air.
Another prior art approach to limiting cathodic potentials in order to avoid potential hydrogen embrittlement has been to use voltage-limiting diodes in series electrically between the sacrificial anode and the protected structure. This approach has the advantage that standard marine aluminum-zinc-indium anodes may be used, but it has several disadvantages in service, including (a) the sacrificial anode must be isolated electrically from the protected structure, (b) the diodes constitute an additional potential failure point in the system, and (c) the cathodic potential in the protected structure may not be adjusted, (d) the break-down voltage of the diodes may not be exactly correct, or it may be quite “sharp” or “abrupt” where, in a CP system, a more gradual break-down may be desired, and finally (e) such a system may be highly inefficient, as at least some exposed anodic area may not be electrically connected to the protected structure (that is, anodes may be corroding-away in an open-circuit condition without providing any cathodic protection).
One passive sacrificial anode of the prior art, as taught in European Patent Application EP 0615002A1 (“the EP-002 application”) from AGIP S.p.A. of Milano, Italy, is shown in FIG. 1. It is designed to apply a “potential profile” over time by the expedient of a composite anode structure using two anodic materials; conductive carrier means 1 has an over-molded inner core 2 of anodic material with higher electronegativity than the structure to be protected (that is, a relatively less noble material, such as an aluminum alloy), with an outer coating 3 of anodic material with a still higher electronegativity than inner core 2 (that is, an even less noble material, such as a magnesium alloy).
Initially, the outer coating 3 of, say, magnesium alloy will induce a relatively high negative cathodic potential, which has been shown experimentally to favor the creation of a dense base layer of protective calcareous deposits on the protected structures and equipment. Subsequently, after the outer coating 3 is sacrificially consumed, a relatively lower negative cathodic potential will be induced by the inner core 2 of, say, aluminum alloy.
As will be clear to those with ordinary skill in the art, this approach requires careful selection of such variables as the anodic materials, placement of the sacrificial anodes on the protected structure, and the thickness of the outer anode layer, in order to achieve the desired potential profile. Further, it is not contemplated that either the cathodic potential or the current density created by this composite anode be adjustable in situ. In addition, the present inventors of the current disclosure believe that it would be difficult to achieve a uniform and structurally and electrically sound interface between the two anodic materials of this design. The inventors of the current disclosure believe that because of these and other limitations of this design, the anodes taught in the EP-002 application are not commercial available.
A prior art means of continuously providing sacrificial anode material to a protected structure is taught in U.S. Pat. No. 4,549,948 (the '948 patent) issued to Peterson, et al, is shown is FIG. 2. Note that a similar system is taught in related U.S. Pat. No. 4,318,787, issued to the same inventors. FIG. 2 shows a cross-sectional view of a container 12 attached to a support member 11 of an offshore platform (not shown). The sacrificial anode may be replenished continuously or periodically by feeding the anode in particulate form to the container 12, which is located under the surface of the water and electrically connected to the structure (offshore platform) to be protected. As shown, the container 12 has perforations 23 to allow the water to enter it. It is attached to support member 11 by a support bracket 15 and the lower portion of the container is an extrusion die 24 made of steel. A thixotropic mixture of a thixotropic carrier material and particulate anodic material is pumped from the surface through conduit 13 into extrusion die 24; under pressure from the surface, the thixotropic mixture extrudes from extrusion die 24 into elongated shape 26 within subsea container 12 forming a sacrificial anode in contact with seawater (the electrolyte) passing through perforations 23. A separate electrical connection 27 is provided if necessary to provide electrical continuity between the container 12 and the protected structure, which includes support member 11.
Although not contemplated in prior art, including in U.S. Pat. No. 4,549,948, or related U.S. Pat. No. 4,318,787, this device could be used with a variety of thixotropic mixtures comprising particulate anode materials of different electronegativities in order to produce a “potential profile” over time, by, for example, initially pumping a thixotropic mixture with high electronegativity and later in time, after say a dense calcareous layer had developed on the protected structure, changing the thixotropic mixture pumped to a mixture with lower electronegativity.
Similarly, although also not contemplated in the prior art, CP current created by the apparatus taught in U.S. Pat. No. 4,549,948 could be changed by adjusting the surface area of the elongated shape 26, by, for example, changing the flow rate of the thixotropic mixture 29 such that the elongated shape 26 is larger and has more surface area, and consequently induces a higher cathodic current, or is smaller and induces a smaller cathodic current.
In practice, however, this anodic thixotropic mixture system has not proven to be practical; the system is inherently complex and expensive, and offers no particular advantages over an impressed current system. Generally, if it is possible to deploy a conduit 13 from the surface to feed an anodic thixotropic mixture 29 to one or more extrusion dies 24, it will likely be cheaper and more effective to deploy electrical cables as part of an impressed current system.
What is needed are passive marine cathodic protection systems and methods which can employ readily available and inexpensive sacrificial anodes, such as aluminum-zinc-indium alloy anodes, and which allow accurate in situ adjustment of the cathodic potential and/or current density of the system, particularly for equipment and structures which can not economically be protected by an impressed cathodic protection system, such as structures and equipment deployed proximate the seabed.